The invention disclosed herein relates generally to serial impact printers which utilize a multi-character printing element mounted on a translatable print carriage, and more particularly to an interchangeable rotary print element and drive coupling apparatus therefor, and components thereof such as a print wheel hub, radial elements, motion restraining structures, and to the formation of such a rotary print element and components thereof.
Business today is increasingly relying on electronic information handling systems. Typically, serial impact printers are used as part of such systems to obtain hard copy. Since serial impact printers print at speeds significantly below the information flow rate available from most modern business information systems, the wait for hard copy usually constitutes a significant, if not the single most significant, factor in obtaining hard copy output from such systems. This is particularly true for modern information handling systems such as word processors, which typically utilize a visual image on a display unit for editing or changing a document. It is therefore not difficult to rapidly edit a very long document, particularly if only a few pages are to be changed. Once the desired changes have been made using the display unit, and the document reformatted if necessary, there is a need to quickly obtain hard copy of the revised document.
There is an accompanying need in business offices for local production of documents with particularly high print quality, such that when optically copied, as is now common in business office practice, the output copies present a highly readable and attractive appearance after any image degradation in the copying process.
There is thus the need to provide high quality hard copy at increased printing speeds.
The faster type of correspondence-quality printers which are commercially available are generally of the impact type, and use a rotating print element having radial elements (commonly referred to as "spokes" or "spoke") extending therefrom each of which carries at least one fully-formed character face. As used herein, the term radial element and spoke or spoke element are used essentially interchangeably and in an unlimiting sense to refer to an element which may be of almost any shape extending from a central element such as a hub or ring. As used herein, the term "character" includes letters, numbers, mathematical and other symbols, etc., and is not used in a limiting sense The rotatable print element is conveyed on a carriage which delivers the print element to each location where a character is to be printed. The print element is in turn rotated to bring the selected spoke with its character face into a printing position. Thus, the print element experiences at least two types of motion, rotating motion for bringing a selected character into a print position with respect to the carriage, and generally lateral motion for positioning the carriage at the appropriate location where the character is to be printed. If the productive output, as measured by printing speed, is to be increased, then the rate at which either and preferably both such motions are performed must be increased.
In most commercially available printing arrangements for serial impact printers utilizing a rotary print element, a motor and means coupling the motor to the print element provide for rotation of the print element to position the desired character for printing. Inasmuch as the rotary element need only traverse a fraction of a revolution in either direction to bring the desired character into position, the major performance criterion is that of rotary acceleration and deceleration times encountered in traversing the required angular distance between initial and final positions. Since a driving motor for the print element must accelerate its own internal rotor and the rotation coupling means, considerable effort has been expended in attempts to keep motor and coupling means rotary moment of inertia to a minimum. Moreover, since the motor must also accelerate and decelerate the rotary print element, efforts have also been expended to reduce the rotary moment of inertia of the rotary print element itself.
Heretofore rotary print elements having the capability of high print quality also had an overly-large rotary moment of inertia, which restricted the accelerational motions available from a given size motor and coupling means combination. On the other hand, with lighter weight print elements, print quality was not entirely satisfactory. Earlier commercial rotary print elements were formed by casting the print element in one piece using durable, lightweight plastic. In order to improve the print quality of such print elements, many character elements, particularly those intended for printing on one or two ply paper, were made with a minimal surface relief from an unreinforced phenolic thermosetting resin. Such resins are known to have good impact strength and a printing life of many millions of impacts. Some of the all-plastic print elements were made of an injection molded thermoplastic, such as a type 66 Nylon available from A. L. Hyde Company, Grenloch, New Jersey, to which phonolic resin character elements were affixed. It was known to metal plate molded R phenolic resin character faces, but essentially for cosmetic reasons to give the molded character faces a pleasing metallic appearance. Later commercial units for very high quality work utilized a plastic hub into which were cast metal spokes with metal-plated plastic character tips affixed to the ends of the spokes.
For slower speed print elements which can tolerate a higher rotary moment of inertia, the character face can be of metal rather than plastic, formed by press swagging, electroforming, or other methods. However, such metal character face print elements were as a general matter used for specialized printing, for example, where thick, multicopy sets are printed, as in transport waybills, or where long periods of printing on abrasive surfaces are required, as in direct production of dual text containing visible text and braille embossing.
Spoke tips comprised of a character element or face and a rear pad, simultaneously cast together from two mating halves of a plastic molding die are also known. The rear pad may also bear a raised or indented portion which engages a mating surface on the nose area of an impact element or hammer, which acts to stablize the hammer impact, and in some cases to align the character pad centerline along the fixed hammer path. The rear strike pad also, in a limited manner, distributes the impact over the character face. This pair of plastic elements, a rear pad and a front face, were usually cast simultaneously in a mating mold set. One mold half was used to cast all pads, and the other mating mold half was used to cast all of the character faces, the two halves of the mold thereby integrally shaping the spoke tip as a unit. After forming, the character faces were then metallicly plated. While the metallic plating improved print quality and increased the life of the print element, the presence of a large, plated plastic mass at the outer tips of the spokes constituted a major component in the overall rotary moment of inertia of the print element.
There is thus a need to provide an improved spoke tip for a rotary print element which is easy to manufacture, and/or which provides good print quality, and/or which does not add to, and preferably reduces, the rotary moment of inertia of the print element and/or which has good service life.
Despite the improved quality, the metal-plastic print element had an increased rotary moment of inertia that tended to slow down the printer. For example, when an all plastic print element was used in a very high speed serial printer, the observed speed of the printer had burst speeds of 80 characters per second, but when a standard metal-plastic composite print element was used so as to achieve better print quality, the observed burst speed was reduced to 63 characters per second because of the higher rotary moment of inertia of the composite print element. Although it may have been possible to increase the burst speed of this particular printer with the composite print element by increasing motor accelerational capability, to do so necessitated the use of a much heavier motor which would, in turn, significantly encumber the translational speeds available. Increasing the size of the translational motor could compensate for the reduction in translational speed. However, for the particular printer, the translational linkages and guideways would have to be increased accordingly. Thus, simply increasing motor size to increase printing speed was not a satisfactory solution.
There is thus a need to provide print elements capable of high rotary and translational motions while producing high quality print, and to accomplish this without simply increasing motor size and linkage mass.
In order to make characters of differing languages or fonts available to the printing equipment operator, it is desirable to allow interchange of rotary print elements. Commercially available rotary print elements of which the most commonly used is known as a print wheel, are generally interchangeable if they have the same base configurations, i.e. the same hub configuration and the same angular subtent between spokes. Another requirement for interchangeability is that interchangeable print wheels, whether of the fixed or proportional spacing type, emplace the characters at identical vertical center lines. For manual interchange systems, it is also highly desirable that the steps needed for an interchange be relatively few and straightforward. Further, since most printing systems use a known reference line and some type of relative shaft angle encoder to deliniate the available character positions, each print wheel must only be emplaced with this same reference line. Interchangeable print wheels must also have compatible character position identifications and compatible hammer blow intensities for respective character positions, and that information regarding such operating characteristics be affixed to the print wheel so as to be manually identifiable by the operator. Such character set information and reference line validity enable a shaft encoder to deliver correct information as to spoke position to allow printing of desired characters.
Some known print wheels are permanently incorporated in a thin box, referred to as a containment box, that is itself inserted into the printer. This box allows the print wheel to be rotated by the system to find and lock-in the correct centerline. While the use of the print wheel box eliminates from the print wheel some of the rigors of manual handling, it does so at the cost of yet more translatable mass. In addition, known locking means may require substantial hub material at the inner radius, or root of the spokes, so as to transfer the high accelerational forces to the spokes which carry the character tips.
There is thus a further need to provide interchangeable print elements which are easy to change and yet are capable of quality, high-speed printing.
A composite print wheel, while delivering excellent print quality, has been so expensive as to restrict the range and type of print wheel use, thereby reducing full utilization of all the capabilities inherent in the rotary print wheel type of fast serial printer. The unitary, cast all-plastic print wheel has the virtue of simplicity in manufacture and thus can be provided at low cost, thereby encouraging the acquisition of a library of print wheels having different fonts and perhaps different language capability. In contrast, the composite metal-plastic print wheel requires over a dozen fabrication steps, many of high precision, so that the manufacturing cost of a high quality composite print wheel is an order of magnitude higher than the all-plastic print wheel. Also, because of the manufacturing complexity and cost, it is a known problem with the composite print wheel that the margin between manufacturing cost and sales price is much less than usual, so that supplies vendors usually only stocked the faster-moving, standard print wheels. On the other hand, supplies vendors who could provide other than standard print wheels charged more for the slower-moving special types, so that many users were unlikely to acquire a particularly extensive library of special print wheels. Thus, unless an urgent need for a special print wheel had been clearly demonstrated, it was unlikely that anything but a standard type print wheel could be obtained from stock. The tendency of the user has therefore been to ignore or forget the possibilities of performing variable font printing that could enhance the visual appearance of the output copy, and to be inclined to forget that the printer may have been capable of doing certain specialized tasks, such as mathematical symbol printing.
There has also been the need heretofore to customize a print wheel with a very special character set, perhaps having an unusual symbol, such as a firm's logo or an infrequently used mathematical symbol. When the total production run of such a special print wheel was small, it was uneconomical to make a special mold of the entire character set. As indicated above, existing composite print wheels have tip units which consist of two parts simultaneously cast to form one unit, a rear pad and a front strike face on which the raised character is placed. It is known to mold single characters in special mold units. These special mold units usually have a central cavity in the form of a slot which matches the dimensions of the spoke tip and are usually made to fit a spoke that already bears a little used special character, for example, the ampersand sign "&", which often appears as the upper case of the figure "7". The previously cast ampersand may be removed, after which a special character may be adhesively fixed. To aid such adhesive fixing of special characters, it was known to provide the spoke tip with a central hole to contain a glue droplet for best retention of the special character unit. (It was also known to use such holes in group casting to help affix the character element to the tip by a measure of through hole plastic casting.) A glue, such as a cyanoacrylate, can give good immediate retention, but the longer term glue bond properties occasionally lead to retention failures under heavy hammer impact blows. It is well known, however, that these special character emplacement methods cannot be used for a single-pass, all plastic casting in which all character elements are simultaneously formed.
There is thus a further need to provide special character set print wheels economically, preferably with good print quality and capable of high speed printing.